STRUCTURE 541 



one group to accomplish some specific function it is called a nerve center. 

 The term usually applies to the aggregation of cell bodies and their dendritic 

 processes in contradistinction to nerve trunks. There are aggregations 

 of nerve cells into different specific groups, to which we cannot in every 

 case ascribe a specific function. These groups are not called nerve centers, 

 but are described by the general anatomical term, ganglia. Such ganglia 

 are represented in the sympathetic chain, the spinal-root ganglia, the ganglia 

 of certain cranial nerves, etc. The nerve centers are found in the spinal cord, 

 the medulla, and the higher cranial groups. The medulla is particularly 

 rich in nerve centers. The cerebro-spinal axis is in fact an aggregation of 

 nerve centers of varying complexity in different parts. 



It is by means of the nerve centers that the activities of the differentiated 

 parts of the human body are brought into intimate correlation. The nerve 

 centers exercise their influence through the power of inhibiting or decreasing 

 activity; or, on the other hand, of augmenting or increasing the activity in the 

 peripheral tissues or in other parts of the nervous system. For example, the 

 vagus center regulates the activity of. the heart muscle by its power to decrease 

 or inhibit cardiac contractions. This center, we have already found, is in 

 constant tonic activity; that is to say, in constant regulative control of the 

 heart. The cardiac augmentory center, on the other hand, produces just the 

 opposite effect, increasing the activity of the cardiac muscle. What is true 

 for the heart is likewise true in general for other tissues of the body. The 

 numerous nerve centers in the central nervous system are brought into cor- 

 relation through an exceedingly complex system of communicating fibers. 

 The cerebro-spinal axis may in fact be regarded as a segmental chain of nerve 

 centers, the complexity increasing from the cord toward the brain, and the 

 coordinating control culminating in the cerebral cortex. 



II. THE STRUCTURE AND FUNCTION OF THE 

 SPINAL CORD. 



STRUCTURE. 



The spinal cord is a cylindrical column of nerve-substance connected 

 above with the brain through the medium of the bulb, and terminating below 

 in a slender filament of nerve substance, the filum terminate, which lies in the 

 midst of the roots of the many nerves forming the cauda equina. 



General Features. The cord is composed of nerve fibers and nerve 

 cells. The former are situated externally and constitute the chief portion 

 of the cord, while the latter occupy its central or axial portion and are so 

 disposed that on the surface of a transverse section of the cord two somewhat 

 crescentic grayish masses connected by a narrower portion or isthmus ap- 

 pear, figure 358. Passing through the center of the cord in a longitudinal 

 direction is a minute canal, the central canal, which is continued through 



